The invention concerns a valve actuation mechanism for an internal combustion engine on an automotive vehicle. The invention also concerns an automotive vehicle, such as a truck, equipped with such a valve actuation mechanism.
Automotive vehicles, such as trucks, often rely on an engine brake function to slow down in order, for example, to reduce wear of the friction brake pads and to prevent overheating of the friction brakes, particularly on downward slopes. It is known to perform engine brake by acting on the amount of gas present in the cylinders of the engine in two distinct phases. In a first phase, when the pistons are near a bottom dead center, one injects exhaust gases into the chambers of the cylinders so as to slow down the pistons when they move towards their high level. This is done by slightly opening at least a valve connected to an exhaust manifold, while exhaust gases are prevented to be expelled from the exhaust pipe and thereby at a certain pressure above atmospheric pressure. In the second phase, the gases which are compressed the piston are expelled from the chamber of the cylinder when the piston is at or near its top dead center position in order to prevent an acceleration of the piston under effect of volcanic expansion of compressed gas, this is done by slightly opening a valve so as to expel gases from the cylinder. In most cases, the valve (or valves) which is (are) opened for the engine brake function is (are) a main exhaust valve. An engine brake system is described in document WO 9009514.
To perform these engine brake valves movements, also called engine brake valves lifts, the engine comprises, for each cylinder, a rocker acting, on the valves to open and close them. The rocker is acted upon by a rotating cam which has at least one lift sector to cause the lifting (opening) of the valve. If the valve is also an exhaust or an intake valve, the corresponding cam will comprise a main valve lift sector and one or several auxiliary valve lift sectors (also called main valve lift bum When engine brake is needed, a cam follower surface of the rocker is moved in close contact with a cam of a camshaft moving the rocker so that the brake movements of the valve are obtained, when the cam follower interacts with the auxiliary valve lift sectors. In normal operating conditions of the engine, the valves should not perform these movements and the roller of the rocker is kept slightly remote from the cam so that the cam follower does not interact with the auxiliary valve lift sectors. The distance or clearance between the roller and the earn ensures that only the larger main lift sector on the cam, dedicated to the main exhaust event, causes an opening of the exhaust valve, but not one or several smaller auxiliary lift sectors dedicated to the engine brake function. This clearance is suppressed when engine brake is needed, by moving an activation piston of the rocker to make a close contact between the roller and the cam, so that engine brake dedicated lift sectors on the cam also cause an opening of the valve. An engine brake system having such valve actuation mechanism is described in WO-91/08381
In the case of a system where two valves are to be actuated, the piston can be in contact with the valves through a valve bridge.
When the engine brake valve opening(s) have been performed, a reset function is preferably to be performed. In other words, the activation piston needs to be moved towards its initial position in order to ensure that the valves are closed early enough in order to prevent extended valve lift overlap.
Engine brake systems generally comprise a control valve to direct pressurized control fluid pressure in a chamber adjacent to the piston to move the activation piston from its initial position to its engine brake actuation position. The control valve controls whether or not the engine brake function is activated. This control valve lets pressurized, control fluid flow, at a pressure of for example 2 to 5 bars, towards each rocker as long as the engine brake function is needed, which typically lasts several seconds or tens of seconds during which the engine and the cam shaft may perform several hundreds or thousands of complete revolutions. In some systems, a check valve is provided to prevent any fluid flow out of the chamber. In some known systems, such as the one described in WO-91/08381, the check valve can nevertheless be forced to an open position, allowing the control fluid to escape the chamber when the engine brake is not needed. This is achieved when no control pressure is sent to the control valve. In known systems, there is only one control valve for several cylinders, so that it is not possible to use the control valve to empty the chamber to allow retraction of the piston, if such retraction is needed for a period of time inferior to one revolution of the camshaft.
It is known, for example from U.S. Pat. No. 6,253,730, to act on the check valve thanks to a stopper which is fixed to a housing, of the engine, so as to open the check valve and release fluid pressure in the chamber so that the piston may move towards its initial position, retracted. This technical solution is not applicable in the case of a so-called “single valve engine braking” where the additional valve lift opening are performed with only one of two exhaust vales is opened for performing engine braking. Indeed, the stopper has to be positioned with respect to the rocker so that it forces the check valve to an open position for a valve lift value superior to the additional valve lift value, but allows the check valve to close again at the same valve lift value when the valves are closing, allowing the actuation piston to be extended again, which delays the valve closing.
The aim of the invention is to provide a valve actuation mechanism in which the fluid pressure in the piston chamber can be reduced with satisfying time accuracy and relatively low forces.
To this end, the invention concerns a valve actuation mechanism for an internal combustion engine on an automotive vehicle, comprising rockers moved by a camshaft, each rocker being adapted to exert a valve opening force on at least a portion of a opening actuator of each cylinder, via an activation piston of the rocker movable with respect to the rocker under action of a fluid pressure raise in a chamber, from a first position to a second position, in which a cam follower of the rocker is adapted to read at least one auxiliary valve lift sector of a cam of the camshaft so as to perform an engine operating function, each rocker comprising a valve for releasing fluid from the chamber, wherein the valve actuation mechanism comprises, for each rocker, a stopper fast with a housing of the engine and adapted to exert, on a member of the rocker, a variable force for opening, the fluid releasing valve.
According to further aspects of the invention which are advantageous but not compulsory, such a valve actuation mechanism may incorporate one or several of the following features:                the variable force increases when the rocker rotates from a valve closing position to its valve opening position;        the stopper causes opening of the fluid releasing valve for a first position of the rocker and allows closing of the fluid releasing valve for a second position of the rocker, said second position being closer to the valve closing position of the rocker than said first position;        the stopper comprises elastic means which are stressed when the rocker travels from its valve closing position to its valve opening position;        the stopper comprises a spring adapted, when deformed, to exert a compression force on said member;        the stopper comprises a mobile contact element biased by the spring and adapted to cooperate with said member, the contact element and the spring are movable in translation with respect to a jacket in which the contact element and the main spring are housed, said jacket being fast with said engine housing.        the jacket comprises a stop element against which the contact element comes in abutment when the piston has to be moved from its second position to its first position;        the elastic means of the stopper have a variable stiffness;        the stopper comprises a main spring and an auxiliary spring, wherein, during a first portion of the rocker travel from a valve closing to a valve opining position, only the auxiliary spring is stressed, and wherein during a second portion of the rocker travel, the main spring is stressed;        the stopper is in permanent contact with the member of the rocker on which the force of the stopper is exerted;        prior to the exertion of the force of the stopper on the member of the rocker, the stopper is remote from the member by a clearance;        
the force exerted by the stopper on said member is adapted to overcome a force keeping said valve in a closed position only when the piston has to be moved from its second position to its first position;                for each rocker, the member on which the force of the stopper is exerted cooperates with a check valve adapted to allow fluid flow from a fluid feeding, circuit of the rocker to the chamber or to block fluid flow from the chamber to the fluid feeding circuit, said check valve forming the valve for releasing fluid from the chamber.        for each rocker, the member on which the force of the stopper is exerted cooperates with a reset valve, movable with respect to the rocker, between a first position, in which it blocks fluid flow between the chamber and the outside of the rocker, and a second position, in which it allows fluid flow between the chamber and the outside of the rocker, said reset valve forming the valve for releasing fluid from the chamber;        the fluid releasing valve is adapted to allow fluid flow from the chamber to the outside of the rocker, wherein the piston) comprises:        a first element housed in the bore and movable in translation with respect to the rocker,        and a central member housed in a portion of the first element and movable in translation with respect to the first element along a longitudinal axis of the piston,        wherein the fluid releasing valve is formed by a cooperation between the first element and the central member, and wherein the force of the stopper is exerted on the first element.        the valve for releasing fluid from the chamber is kept in its closed position by a fluid pressure force in a chamber fluidly connected to the piston chamber;        each rocker comprises a normally closed discharge valve which is opened by the fluid pressure in the chamber when such pressure exceeds a predetermined threshold to allow fluid flow out of the chamber, said discharge valve forming the valve for releasing fluid from the chamber, and wherein the member on which the force of the stopper is exerted is the piston;        the discharge valve is carried by the piston;        the valve for reducing fluid pressure in the chamber is biased towards its closed position by a spring;        the valve actuation mechanism is one of:        an exhaust valve actuation mechanism:        wherein the activation piston (95) activates an exhaust gases recirculation function when it is in its second position; or        wherein the activation piston (95) activates an engine brake function when it is in its second position; or        an intake valve actuation mechanism.        
The invention also concerns an automotive vehicle, such as a truck, comprising a valve actuation mechanism as mentioned here-above.